What might be cause of sudden left turn steering problem and how to duplicate this issue?

[jani12]

Please consider commerical truck with Magnetic Torque Overlay (MTO) Hydraulic Active Steering System that includes Lane Keep Assist (LKA).

Vehicle experienced a sudden left turn steering problem. It went off to the right side of the roadway. Afterward:
1. There were no active fault codes with the active steering system.
2. During the test drive, the vehicle has a light steering effort.
3. The LKA would only warn when vehicle was departing from it's lane. It wouldn't automatically force vehicle to center of the lane.
4. The power to the MTO steering gear was disconnected to create a steering fault, and then the power was reconnected with the key cycle to correct the fault. Afterwards, steering feel was firmer and when vehicle drifts out of the lane, LKA would force vehicle back into the lane.
5. Steering angle sensor appears to be working properly.

Is this a steering system issue?
What might be the cause of the problem?
How to duplicate this problem?

My theory is that diagnostics in steering ECU are working fine. The cause might be that engineers missed identifying all the faults. Some unidentified fault occurred. ECU software and hardware didn't check for this fault. Therefore, there were not active fault codes. Does this theory make sense?

 

[mbarazeen]

I assume it is a hydraulic servo system to turn the wheels based on steering wheel position. The issue may be wrong feedback. if feedback fails the system will over turn. Another issue to check is electric servo valve (LVDT) if any. this should receive signal from ECU (current signal) to adjust the steering to required position. fault code is not expected in both cases.

 

[jani12]

I assume it is a hydraulic servo system to turn the wheels based on steering wheel position.

Yes, it's hydraulic steering system but it's Magnetic Torque Overlay (MTO) system. There is no motor.

 

[rjenkinsgb]

it's Magnetic Torque Overlay (MTO) system.

That means that in effect it has a brushless torque motor built in to the steering column hydraulic torque sensing valve system, so it can emulate the driver putting force on the wheel.

The fault could be mechanical, such as grit (dirt, stray particle of something) in the hydraulic valve, preventing it moving correctly. Once pressure and power were removed so the valve was not being forced to a position, the grit could have moved and later been passed straight through to somewhere less critical, or have been caught in a filter.

Or such as a water splash temporarily affecting wires in an improperly sealed connector, or a sensor with a flawed casing.

A corroded connector pin is also a possibility. Someone moving wires while inspecting the system could inadvertently "cure" the fault - temporarily.

ECU diagnostics can usually detect bus faults or open / short circuits in devices, but often not such as contamination or poor connections that affect sensor readings without causing them to exceed their working limits.

These are all classes of faults that can and do occur in real world vehicle systems.